The invention concerns a method for low coherence interferometry of a biological sample using phase. The term “biological sample” denotes a body fluid or tissue of an organism. Biological samples are generally optically heterogeneous, that is, they contain a plurality of scattering centers scattering irradiated light. In the case of biological tissue, especially skin tissue, the cell walls and other intra-tissue components form the scattering centers.
Generally, for the qualitative and quantitative analysis in such biological samples, reagents or systems of reagents are used that chemically react with the particular component(s) to be determined. The reaction results in a physically detectable change in the solution of reaction, for instance a change in its color, which can be measured as a measurement quantity. By calibrating with standard samples of known concentration, a correlation is determined between the values of the measurement quantity measured at different concentrations and the particular concentration. These procedures allow accurate and sensitive analyses, but on the other hand they require removing a liquid sample, especially a blood sample, from the body for the analysis (“invasive analysis”).
The American Diabetes Association (ADA) estimates that diabetes afflicts nearly 17 million people in the United States. Diabetes can lead to severe complications over time, including heart failure, kidney failure, blindness, and loss of limb due to poor peripheral circulation. According to ADA, complications arising from diabetes cost the U.S. health care system in excess of $132 Billion.
Diabetes complications are largely due to years of poor blood glucose control. The Diabetes Care and Complications Trial (DCCT) carried out by the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) demonstrated that more frequent monitoring of blood glucose and insulin levels can prevent many of the long-term complications of diabetes.
Monitoring of blood glucose concentration is key to managing the therapy of diabetes patients. Monitoring results are used to adjust nutrition, medication, and exercise in order to achieve the best possible glucose control, reducing the complications and mortality associated with diabetes. At present, the most widely used method for monitoring of blood glucose by diabetes patients involves chemical analysis of blood samples taken by puncturing the finger or forearm. This method is painful, requires relatively complex operations, is inconvenient due to disruption of daily life, and may become difficult to perform in the long term due to calluses on the fingers and poor circulation. As a result, the average diabetic patient tests his/her blood glucose levels less than twice a day versus the recommended four or more times per day. Non-invasive blood glucose monitoring techniques with accuracies equal to or better than the current chemical glucose methods are therefore needed.
Accordingly, a number of procedures and apparatus have been suggested to determine glucose in blood, tissue and other biological samples in vivo and in a non-invasive manner. Existing non-invasive procedures for glucose determination include nuclear magnetic resonance (NMR), electron spin resonance (ESR) and infrared spectroscopy. However, none of these procedures have achieved practical significance. Large and costly equipment is required, which are wholly unsuitable for routine analysis or even for patient self-checking (home monitoring).
One of the most promising approaches for non-invasive glucose monitoring is based on optical techniques. Optical glucose monitoring techniques are particularly attractive in that they are relatively fast, use non-ionizing radiation, and generally do not require consumable reagents. Several optical glucose monitoring techniques have been proposed so far, with varying degrees of success. Several of these techniques are discussed herein as background, however, once again, none of these techniques has attained significant commercial success relative to invasive techniques.
One approach is Near-Infrared (NIR)/Mid-Infrared (MIR) spectroscopy. In infrared spectroscopy, radiation from external light sources is transmitted through or reflected by a body part. Spectroscopic techniques are used to analyze the amount of radiation absorbed at each wavelength by the body part constituents and to compare the absorption data to known data for glucose. Practical implementation of a glucose sensor based on these principles is very difficult and several wavelengths are required. Infrared (IR) spectra are sensitive to physical and chemical factors such as temperature, pH, and scattering. Furthermore, spectroscopy is affected by skin pigmentation, use of medications that absorb various IR wavelengths, alterations in blood levels of hemoglobin or other proteins that absorb IR, changes in body temperature, and alterations in the state of hydration or nutrition. In addition, the NIR spectrum of glucose is very similar to that of other sugars, including fructose, which is often used by diabetics. Therefore, the signal (i.e. the change in the absorption spectrum as a function of glucose concentration) is very small compared to noise and to interference resulting especially from the water spectral absorption and other strongly absorbing components.
Another approach is Raman Spectroscopy. With Raman spectroscopy, Raman spectra are observed when incident radiation is inelastically scattered. The loss or gain of photon energy are independent of the excitation frequency and provide specific information about the chemical structure of the sample. The Raman signal is very weak, requiring long data acquisition time, making the device sensitive to light source fluctuations. Measurements are subject to high background noise because of tissue autofluorescence. Scatter and reabsorption in biological tissues make detection of Raman frequency shifts due to physiological concentrations difficult.
Another spectroscopic approach is based on photoacoustics. In photoacoustic spectroscopy, a laser beam pulse is used to rapidly heat the tissue and generate an acoustic pressure wave that can be measured by a microphone or other transducer. The acoustic signal is analyzed to infer blood glucose concentration. Measurements are affected by chemical interferences from biological molecules as well as physical interference from temperature and pressure changes. Current instruments are complex and sensitive to environmental conditions.
Another optical approach considered of glucose monitoring is based on employing polarimetry. Glucose concentration changes the polarization of light fields. The eye's aqueous humor has been suggested as the medium for this technique as skin is not a feasible site due to its high light scattering properties. However, polarization measurements are affected by optical rotation due to cornea, and by other optically active substances. Other interfering factors include saccadic motion and corneal birefringence. In addition, there is a significant lag between blood glucose changes and glucose changes in intra-ocular fluids, of up to 30 minutes.
Yet, another approach employed for glucose monitoring is based on light scattering. Changes in glucose levels induce changes in light scattering properties, generally, of the skin. U.S. Pat. No. 6,226,089 to Hakamata discloses detecting the intensities of backscattering light generated by predetermined interfaces of an eyeball when a laser beam emitted from a semiconductor laser is projected onto the eyeball in a predetermined position. The absorbance or refractive index of the aqueous humor in the anterior chamber of the eyeball is determined on the basis of the intensities of the backscattering light, and the glucose concentration in the aqueous humor is determined on the basis of the absorbance or refractive index in the aqueous humor. Light scattering effects are evident in the near-infrared range, where water absorption is much weaker than at larger wavelengths (medium- and far-infrared). However, techniques that rely on the backscattered light from the aqueous humor of the eye are affected by optical rotation due to cornea, and by other optically active substances. Other interfering factors include saccadic motion and corneal birefringence. Finally, it should be appreciated that there is often a significant time lag, (e.g., up to 30 minutes) between blood glucose changes and glucose changes of the intra-ocular fluids.
Low-Coherence Interferometry (LCI) is one technique for analyzing skin light scattering properties. Low Coherence Interferometry (LCI) is an optical technique that allows for accurate, analysis of the scattering properties of heterogeneous optical media such as biological tissue. In LCI, light from a broad bandwidth light source is first split into sample and reference light beams which are both retro-reflected, from a targeted region of the sample and from a reference mirror, respectively, and are subsequently recombined to generate an interference signal. Constructive interference between the sample and reference beams occurs only if the optical path difference between them is less than the coherence length of the source.
U.S. Pat. No. 5,710,630 to Essenpreis et al. describes a glucose measuring apparatus for the analytical determination of the glucose concentration in a biological sample and comprising a light source to generate the measuring light, light irradiation means comprising a light aperture by means of which the measuring light is irradiated into the biological sample through a boundary surface thereof, a primary-side measuring light path from the light source to the boundary surface, light receiving means for the measuring light emerging from a sample boundary surface following interaction with said sample, and a secondary-side sample light path linking the boundary surface where the measuring light emerges from the sample with a photodetector. The apparatus being characterized in that the light source and the photodetector are connected by a reference light path of defined optical length and in that an optic coupler is inserted into the secondary-side measurement light path which combines the secondary-side measuring light path with the reference light path in such manner that they impinge on the photodetector at the same location thereby generating an interference signal. A glucose concentration is determined utilizing the optical path length of the secondary-side measuring light path inside the sample derived from the interference signal.